What muscle variable(s) does the nervous system control in limb movements?

Abstract

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Relative phase dynamics in perturbed interlimb coordination: stability and stochasticity

“…Stein 1982)? In this context, controlled variables are often de®ned as those variables that can take on new values independently of other controlled variables as conditions are changed (Latash 1993;cf.…”

Coupling the Neural and Physical Dynamics in Rhythmic Movements

“…The spatial and temporal characteristics of the movement are dictated by central commands or reference signals, which are followed passively by the peripheral musculoskeletal system. In closed-loop schemes, afferent feedback acts to reduce any error between the reference signal and the actual position or velocity of the plant (Merton 1953;Stein 1982). Nevertheless, ~ *Correspondence should be sent to Nicholas G. Hatsopoulos, Department of Neuroscience, Box 1953, Brown University, Providence, RI 02912. the frequency and amplitude of the movement are centrally dictated by a series of set points.…”

Force and Timing Components of the Motor Program

“…However, there are a number of different ways in which these changes can be effected and thus it has remained unclear how a central control system governs movement (see Stein, 1982 for a review of this problem).…”